1. Field of the Invention:
This invention relates to a metal powder used as an electrically conductive filler for an electrically conductive paste and an electrically conductive paste using the metal powder.
2. Background Art:
Powders of silver, copper and other metals have conventionally been used as electrically conductive fillers for electrically conductive pastes. When a conductive paste prepared using a conductive filler is formed into a conductor, the filler is necessary to ensure good retentivity and accuracy of the conductor shape without increasing the conductor resistance. For this, it is generally considered preferable for the particles of the metal powder to be flaky rather than spherical. Flaky metal powder has a large specific surface area owing to the flatness of its particles, so that when the metal powder is used in a conductive paste the contact area between the particles becomes large. This is advantageous from the viewpoint of minimizing electrical resistance and regulating paste rheology.
A conventional method used to produce a flaky metal powder for a conductive filler is to plastically deform the particles of a metal powder obtained by electrolytic deposition, wet reduction, atomization or other such method into a flat shape by crushing the metal powder in a ball mill, bead mill or other such crusher. Related technologies are set out in JP 7-118701A, JP 11-264001A, JP2003-119501A and JP2004-27246A.
The trend in electronic equipment continues to be toward lighter weight and smaller size. This has further increased the need to make the electrodes of electronic components thinner and electronic wiring finer. However, currently available metal powder production methods are limited in the degree to which they can reduce the size of the fine particle flakes. Various problems are encountered when using a ball mill, bead mill or the like for flattening. For example, the setting of the crushing aid conditions and the like is troublesome and the use of balls, beads or other such crushing media makes necessary a process for separating the crushing medium and material powder. Moreover, flattening by the wet method involves additional dry processes such as a drying step, which increases cost. Moreover, when a metal powder flattened using a ball mill, bead mill or the like is utilized, many voids are liable to occur in the conductor obtained by firing the paste.
An object of this invention is therefore to provide a novel metal powder that is obtainable by a production process whose process load is smaller than that by a conventional mechanical crushing means and that when used as a conductive paste filler provides a fired conductor low in voids.